Former UK prime minister Tony Blair has cast doubt on the ability of recent US-led airstrikes to defeat the Islamic State (IS) in Syria and Iraq, arguing in recent comments that the militant group will need to be fought "on the ground".

In an interview with CNN on Sunday, Blair said that the "west must fight not only ISIS" – the former name of the group – "but it's ideology."

The United States and several Arab allies – Jordan, Bahrain, Saudi Arabia and the United Arab Emirates – launched a series of airstrikes early Tuesday morning in Syria against IS and Al-Nusra Front, an Al-Qaeda affiliate, killing at least 50 IS fighters and eight civilians, according to Reuters.

However, while insisting the strikes played a leading role in hampering the militant group, Blair stressed in an essay published on his foundation's website on Monday that "airpower alone will not suffice".

"We are, in certain situations where it is necessary and subject to all proper limitations, talking about committing ground forces, especially those with special capabilities," he said, adding that he wasn't referring to armies of occupation.

He urged for strong action against IS and the "citizens of our own country" that seek to join the group and called for a comprehensive strategy to counter religious extremism in the Middle East to avoid facing a future marked by "conflict and instability."

Describing recent events in Syria and Iraq as "horrifying," he said the violence has shifted the terms of the debate about "whether intervention is desirable or sensible to counter the violence in the Middle East and its potential impact back in our own countries."

The UK government said on Tuesday that no final decision had been made yet over joining air strikes launched by the US and several Gulf Arab allies on IS strongholds in Syria.

A spokeswoman at the British Ministry of Defence said "discussions were still ongoing," Reuters reported.

UK Foreign Affairs Secretary Philip Hammond has said the UK will not be involved in air strikes against IS in Syria.

Blair's term as prime minister was marked by his 2003 decision to send British troops to fight in Iraq along with US troops. Now he says the US coalition against IS "should be given all support."

His essay on Monday also cast doubt on Iraqi and Kurdish fighters' ability in the conflict, and whether they will be sufficient in fighting the militant Islamic group which has seized large swathes of territory in Iraq and Syria in recent months and sparked global concern.

UK Prime Minister David Cameron said at an international conference convened in Paris to discuss the threat of IS earlier this month that Britain would arm Kurdish forces and continue supplying humanitarian aid.

"This is not about British combat troops on the ground," Cameron said. "It is about working with others to extinguish this terrorist threat."